The present invention relates to a signal quality monitoring device in which the phase of a clock signal for demodulating data in a digital communication system is purposely offset to increase the bit error rate (hereinafter referred to as "BER", when applicable), of an input signal and the apparently increased pseudo-error rate is measured to monitor the quality of the input signal.
A device of this type is connected to a receiver as shown in FIG. 1. In FIG. 1, reference numeral 1 designates the input terminal of an input signal applied to the receiver 2; 3, an output terminal of the receiver 2 at which is provided a baseband signal, that is, a signal which has not yet been demodulated; 4, an output terminal at which is provided a recovered clock signal produced by the receiver 2; 5, the signal quality monitoring device; 6, an input terminal for the baseband signal; 7, an input terminal for the recovered clock signal; 8, an output terminal for demodulated data; and 9, an output terminal for an error pulse signal provided for a pseudo-error rate.
FIG. 2 shows in block diagram form an example of a conventional signal quality monitoring device 5. In FIG. 2, 10a and 10b designate sample-and-decision circuits A and B, respectively, which detect at predetermined sampling intervals whether the input signal is "1" or "0", and 11a and 11b denote a phase shifter A and a phase shifter B, respectively, which selectively set the phases of clock signals applied to the sample-and-decision circuits A and B for effecting the sampling operations. More specifically, the phase shifter A (11a) is a variable phase shifter which can set a desired phase shift. Further in FIG. 2, 12a designates an EXCLUSIVE OR gate which receives as inputs the outputs of the samplers A and B.
In operation, the baseband signal is applied through the input terminal 6 to the sample-and-decision circuit A (10a) and the sample-and-decision circuit B (10b). As the signal is not yet demodulated, it can be represented by an "eye" pattern as shown in FIG. 3. In the "eye" pattern of FIG. 3, the transmission waveform is a Nyquist waveform. At the time instant t.sub.0, a "1" or "0" amplitude is transmitted without intersymbol interference from adjacent data bits.
In the receiver 2, at the same time instant to the baseband signal is sampled to demodulate the data. The sample-and-decision circuit A (10a) carries out completely the same operation as the receiver 2. The phase shifter A (11a) operates to adjust the phase of the clock signal applied to the input terminal, thereby to cause the sampling time of the sample-and-decision circuit A (10a) to coincide with the Nyquist point of the baseband signal. Accordingly, demodulated data, which is the same as the demodulated data of the receiver, is provided at the output terminal 8.
The phase shifter B (11b) slightly offsets the phase of the clock signal applied to the sample-and-decision circuit B (10b). Accordingly, the phase shifter B causes the sampler B to act as an offset sampler which performs sampling (demodulation) of the data at a time instant which is somewhat shifted from the Nyquist point. The amount of offset corresponds, for instance, to .DELTA.T in FIG. 3. The value .DELTA.T is, in general, set much smaller than one-half (1/2) of the period T of data. Therefore, the output data of the sample-and-decision circuit B (10b) will always be valid. However, since the sampling time of the sample-and-decision circuit B (10b) is offset by .DELTA.T from the Nyquist point, the BER of the output data of the sample-and-decision circuit B (10b) is larger than that of the output data of the sample-and-decision circuit A (10a). In this sense, the BER of the sample-and-decision circuit B is called a pseudo-error rate. The EXCLUSIVE OR gate 12a is provided to output an error pulse signal which is indicative of the difference between the BERs of the sample-and-decision circuits A and B. The output of the gate is at a low level when the data is demodulated correctly by both of the sample-and-decision circuits 10a and 10b, and at a high level when one of the samples produces an erroneous output, for instance, when the data is demodulated correctly by the sample-and-decision circuit A (10a) but is demodulated erroneously by the sample- and-decision circuit B (10b).
The error pulse signal for the pseudo-error rate is provided at the output terminal 9 as described above. The BER of the received data can be estimated by measuring the frequency of the error pulse signal. That is, even if the content of data is unknown, the quality of the received signal can be monitored by measuring the BER in the manner described.
If the clock signal supplied from the receiver 2 has a phase error .DELTA.t, the sampling time of the sample-and-decision circuit 10a is t.sub.0 +.DELTA.t, and the BER is increased beyond what it would be in the case of no phase error in the clock signal. In such a case, the sampling time of the sample-and-decision circuit 10b is t.sub.0 +.DELTA.T+.DELTA.t. Thus, if .DELTA.T&gt;O and .DELTA.t&gt;0, the pseudo-error rate is increased, and in the case of .DELTA.T&gt;O and .DELTA.t&lt;0, the pseudo-error rate is decreased. In general, the variation of the pseudo-error rate with .DELTA.t is much larger than that of the BER with .DELTA.t, as shown by the graph of FIG. 4. As is apparent from FIG. 4, the relation between the BER and the pseudo-error rate varies as a function of the phase error .DELTA.t of the clock signal, and accordingly, if the BER of the received signal is estimated by measuring the pseudo-error rate, the BER thus estimated includes an error.
Possible causes for the phase error .DELTA.t in the clock signal include:
(1) waveform distortion in the transmission path, PA1 (2) error in the phase shifter A, and PA1 (3) variation of environmental conditions such as the ambient temperature around the receiver and signal quality monitoring device.
In the case of (1) waveform distortion in the transmission path, the relation between the Nyquist point of the received signal and the phase of the clock signal is changed by the waveform distortion of the received signal. For instance, in satellite communications, waveform distortion arises due to nonlinearity in the satellite transponder. If, in an up link to the satellite from a ground station the signal level is changed, for instance, by the presence of rainfall, the operating point of the satellite transponder is changed, and accordingly the characteristic of the signal distortion of the down link is changed. Hence, various phase errors occur in the receiver 2 which depend on the conditions of the up link, which are difficult or impossible to control.
On the other hand, the above-described causes (2) and (3) relate to the device itself. Therefore, the influence due to these causes can be suppressed by carefully performing the phase settings or making provisions for temperature compensation.
An object of the invention is to eliminate the above-described difficulties accompanying a conventional signal quality monitoring device.